Subaortic Membrane

Subaortic Membrane

History: A 55-year-old man had an episode of near syncope. On physical exam he was noted to have a systolic heart murmur. Echocardiogram showed severe left ventricular outflow tract obstruction. The question was raised as to whether the outflow tract obstruction could be due to asymmetric septal hypertrophy from hypertrophic cardiomyopathy. The patient was referred for cardiac MRI for further evaluation.

Technique: The video shows ECG-gated SSFP images. The cine on the left shows a 3-chamber view. The cine on the right shows another view parallel to the left ventricular outflow tract (LVOT).

Findings: The 3-chamber view shows 2 dark systolic jets of blood in the LVOT, compatible with acceleration of blood flow due to LVOT obstruction. The cine on the right shows a thin subaortic membrane even in diastole. There is mild concentric left ventricular hypertrophy (LVH) due to increased left ventricular systolic pressure.

The LVH is not suggestive of hypertrophic cardiomyopathy becuase the hypertrophy is not asymmetric, the maximum left ventricular end-diastolic wall thickness is <15 mm, and there is no systolic anterior motion of the mitral valve leaflets or chordae.

Subvalvular aortic stenosis is differentiated from valvular aortic stenosis because the jet originates in the LVOT rather than at the aortic valve cusps.

Comments: Aortic stenosis can be classified into 3 types: subvalvular, valvular, and supravalvular. Subvalvular stenoses is either be due to a fixed anatomic abnormality (such as from a fibrous membrane or muscular band) or a dynamic functional abnormality (such as from septal hypertrophy as with hypertrophic cardiomyopathy). At surgery the patient was found to have a fibrous band that was resected. Patients with fixed anatomic subaortic stenosis have an increased incidence of other cardiac defects (especially ventricular septal defect). Therefore, it is important to perform a complete cardiac examination in these patients to assess for other possible concurrent abnormalities. In this case, the patient had no other cardiac defects.

Discussion: Subvalvular aortic stenosis (SAS) is a fixed obstruction of LV outflow. Four variants are recognized:

1. Thin fibrous membrane.

2. Thicker fibromuscular ridge.

3. Fibromuscular ring in LVOT and attached to the anterior leaflet of the mitral valve.

4. Fibromuscular tunnel-like narrowing of the LVOT.

Variants 1 and 2 are the most common and comprise approximately 80% of the cases. The clinical significance of the subaortic membrane depends on the degree of LVOT obstruction and the turbulent flow produced by the membrane. The gradient across the membrane may cause LV pressure overload (similar to AS or hypertrophic cardiomyopathy) causing LVH and later ventricular dysfunction, myocardial ischemia, syncope. The flow disturbance of the subaortic membrane can cause the damage to the aortic valve, leading to aortic insufficiency in 65% of patients, and increasing the risk of endocarditis.

Cardiac MRI is a sensitive modality for identification of subaortic membrane. Importantly, CMR is well-suited for the evaluation of the clinical significance of the subvalvular stenosis as well, including the precise location and quantification of the gradient, detection and quantification of aortic insufficiency, and quantitative end qualitative evaluation of LVH and ventricular function.

Reference:

  1. Dahnert. Radiology Review Manual. 1991. Williams and Wilkins. Baltimore.